Molybdenum-zirconium alloys



Patented May 11, 1954 UNITED PAT QFFICE MOLYBDENUM-ZIRCONIUM- ALLOYS J o'hn L. Harp, I'fearporn, Frederick P; Beiis and Alvin J. Herzig, Detroit, and George A Timmons; Ferndale', Mich assignors' to Climax Molybdenumflompany, New York, N-.-Y.;-a cor-- poration ofi Delaware No Drawing; v ApplicationQctober 6', 1951,. Serial No. 250,206

1 3 Claims. (01.7-5-136) The present invention relates" to alloys or molybdenumand zirconium, and more-:sp'ecincally to large'i soun'd cast molybdenum zirconium alloy ingots; which are capable of being worked at elevated temperatures by forging, pressing; rolling, extrusion and other similar methods. This invention is also concernedwith molyb denum-zirconiumalloys containing minor quantities of other elements and to such alloys in which a part of the molybdenumhas' been replaced by tungsten; such anoys are useful in applications which require: high hardness and strength at elevated temperatures, and more specifically in such applications as electrodes for heating'mol-ten glass, die-casting. dies for brass and other metals; etc. This applicationisia con tinuationdn-part'of applicants c'opending application; Serial No. 218,525, .filed March 30;. 1951, now abandoned.

This invention has as its principal". object the characterized by a dispersed carbide phase andrefinedgrairr size;

' The terms casting and cast as used inth-is application are intended to designatethe productresulting from the melting of metal and-solidify ing thesame in a mold, whether or not themetal has been subjected to subsequent working or machining. Ihe term casting is also used to designate any process or method which involves melting. metal and solidifying the same-in a mold.

In accordance with this invention; it is found that: molybdenum=base alloys exhibiting enhanced strengtha'nd hardness are obtainedwhen small quantities of zirconium are used as an alloyingv element. It has been found, however,

that maximum strength isfully realized incast alloys only when the alloy contains limited amounts of oxygen, and that such alloys can beworked at elevated temperatures only if they containlimited." amounts of both-oxygefi and zirconium.

It has previously been established that the presence of minute amounts ofoxygen in a cast-* ing o'f molybdenum or a' molybdenum-base alloyseriously impairs or destroys the capacity of the casting to be worked at elevated temperatures if the oxygen is segregated at the grain boundariesin-the form o'fcer'tain metallic oxides. The detrimental oxide is visible on microscopic exanimation of intergran'ular fractures and is lielieved to consist largely'of'MoOa. However, the oxides-of certain other metals; if present; are also detrimental; many event; when examined microscopically; castings which can be worked at elevated temperatures haveno' similar visible oxide 'segrega'tions atthe grain boundaries which are similar tothe manifestations of M002. Castmolybdenum containing less than about .0019?)- oxygen can be workedat elevated temperatures but it is very difficult inthe-production of casti'ngots of molybdenum andits alloyste reduce the oxygen content (if-the" metal to such a low-- value.

s'et forthin thepatent to Fredericli P. Bens etaL, No; 2,58'0L2'7'3i the detrimental oxide -seg regation is not found i'n molybdenum castings c'o'n tainin'g not more than .00'5-%- ex -gen if smallamounts of carbon are present. Such castingsbe worked at elevated temperatures.

It is now foundthat the detrimental oxides may also be eliminated'b'y i'n'c'o'rpor'ating in the casting certain metals" which have a stronger a'flinity for oxygen than does molybdenum and;

which form an oxide w-l iich either does not'segregate at the gtainboundaries-or, if segregated atthe boundaries provides greater interg ranular cohesion than does the oxide-'- of molybdenum- Aluminum and beryllium have been found tofulfill these requirements, and forgeable casting's of molybdenum and molybdenum-base alloys containing v u'p' to a maximum oi 05% oxygen: have been produced b'y incorporating-small titiesof alilmimlm or beryllium or both the casting; carbon may also-Be present-if desired;- and Small quantities of carbon or aluminum are particularly beneficial in molybdenum-base alloys containing beryllium.

The effect" of oxygen on the mol-ybdenum zirconium alloycastii lgs "of the present invention is sfmilarto its-effect in other'molybdenum base alloycasting's; and consequently it is necessary to eliminate segregations of molybdenum oxide at the grain boundaries if the casting is to be worked at elevated temperatures. This is preferably done by incorporating carbon, aluminum or beryllium in the alloy, either singly or in combination. The critical effect of oxygen on the capacity of the alloy to be worked at elevated temperatures is peculiar to cast alloys as distinguished from those produced by sintering metal powders.

If carbon is present in amounts between 01% and 04% and no aluminum or beryllium is present, the maximum quantity of oxygen which can be tolerated in a casting that must be worked at elevated temperatures is about 005%. Theminimum quantity of residual carbon should, preferably, increase within these limits as the residual oxygen content approaches .005%. Larger amounts of carbon up to a maximum of about 25% may be present in castings that must be worked at elevated temperatures, but the resulting additional carbides increase the difficulty of hot-working the cast alloy without imparting other advantages and, therefore, it is preferred that the carbon not exceed about 07%.

If aluminum or beryllium is present in adequate quantities, the maximum oxygen content which can be tolerated in a casting that must be worked at elevated temperatures 'is I about The quantity of aluminum or beryllium must be at least sufiicient to stoichiometrically react with the oxygen present in the final alloy to form A1203 or Be'Oyand is preferably three times that quantity in the case of aluminum. Thus, aluminum in the range of 003% to 4% or beryllium in the range of 001% to 03% may be present.

, Excellent results are achieved in the working of molybdenum-zirconium castings containing carbon in the range of 02% to 05% and oxygen less than 003% or aluminum from 003% to .2%

and oxygenlessthan 02%; or beryllium from 001% to 02% and oxygen less than .02%. Quantities of aIuminumand beryllium above the minimumrequired to react with the oxygen have other beneficial efiectsand hence aluminum may be present up to a maximum of about 2.5% or beryllium up to a maximum of about 25%. However, as set forth hereinafter, the amount of zirconium present must be .reduced below its maximum if the aluminum exceeds about .4% or the beryllium exceeds about 03% and the alloy is to be worked.

Molybdenum-base alloys containing aluminum or beryllium and the herein-disclosed processof producing such alloys are more fully disclosed and claimed in applicants copending applications, Serial No. 250,202, on Molybdenum-Tungsten-Aluminum Alloys, and Serial No. 250,201, on Cast Alloys and Method for Heat-Treating the Same, both filed concurrently herewith.

The addition of zirconium to cast molybdenum has the following observed effects: (1) it increases the strength and hardness at room temperature; (2) it increases the strength and hardness at elevated temperatures; (3) it refines the grain size of the cast alloy, the refining effect increasing with increasing zirconium content; and (4) it disperses the carbide phase of the cast alloy. It is believed that the increased strength of the cast alloy is due in part to an increase in intergranular cohesion imparted by the zirconium.

Cast molybdenum alloys containing not more than about 2% zirconium may be worked at elevated temperatures to a beneficial degree, but it has been found impractical to work molybdenumzirconium castings containing more than about 2% zirconium. Particularly beneficial alloys are obtained when the zirconium content is kept between .5% and 1.5%, and this range is preferred. The addition of .l% zirconium is effective in improving the high temperature properties of the alloy, but percentages of zirconium below .1% were not found to be particularly advantageous. Alloys of from 0.1% to 2% zirconium in molybdenum are solid solution at room temperature.

-If desired, tungsten may be present in an amount not exceeding the molybdenum present. The substitution of tungsten for molybdenum has the effect of increasing the hardness of the alloy but to a lesser degree than that caused by the addition of zirconium. If the maximum percentage of tungsten is employed in a cast alloy containing the maximum percentage of zirconium, the resulting alloy cannot be successfully worked. However, cast alloys containing tungsten which are capable of being worked at elevated, temperatures are obtained if the zirconium percentage is proportionately decreased toward the minimum of .l% as the tungsten percentage is increased towardits maximum of about 50%. It is to be understood that the relationship between the tungsten and zirconium contents relates only to the maximum allowable zirconium which may be present for a given tungsten percentage without interfering with working, and that alloys containing less zirconium and tungsten than the maximums fall within the scope of this invention. Actually, it is preferred to use no tungsten or amounts less than 10%. Working of molybdenum-zirconium castings at elevated temperatures is facilitated if the oxygen content is decreased toward the practical minimum of about .001% as the amounts of zirconium or tungsten increase.

The useful effects of zirconium in molybdenum-base alloys are realized as long as, in a given alloy, molybdenum is present in an amount exceeding the amount of tungsten present, if any, and the total of the molybdenum and tungsten contents constitutes at least 70% of the alloy. Minor quantities of other elements may also be present. Thus, certain hereinafter listed transition elements have advantageous effects when added to the molybdenum-zirconium alloys of the present invention. However, to produce a cast alloy which can be worked to a beneficial degree, the amounts of tungsten, other transition elements, aluminum and beryllium must be limited; the preferred alloys contain at least molybdenum. Thus, even in pure binary alloys of molybdenum, the following beneficial transition elements should not be present in amounts exceeding the following percentages if the alloy is to be worked:

Per cent Tantalum 9.0 Vanadium 7.0

Berylliumin amounts irr excess of 03% and up to a; maximum of about 25%. and aluminum in amounts in excess of .4% and up to a maximum of2.5'% have an; eflect on workability similar to that of'the' above transition elements. They all producei'a proportionate increase in hardness at I600 F. as their. quantities increase toward the above maximums. The maximum amounts given for beryllium, aluminum. and each of the transition element's other than tungsten correspond'i roughly to those quantities of each element which, when added alone to molybdenum, will produce ahard'ness at 1600" F. of 200 V. P. N. (Vickers pyramidinumeral) in an. annealed casting. It has not beenpossible. with normal workingtechniques to. achieve a worthwhile percentageof recovery fromthe working of metals and alloys. haying. greater hot-hardness, but a benefi'ci'al' working at temperatures substantially above. 1,600 can be. performed on the alloys of the present invention provided the hardness at1600'" F. .does not. exceed about 20.0 V. P. N. in an. annealed casting- The effects of all of the above-mentioned metals, also zirconium and t.ungsten-,. on hot hardness are additive and, therefore, whentwo. are. present, the maximum permissibleamount of oneshould be proportionately reduced from.its maximum. given to the extent that the other approaches its maximum the alloy is to be capable. of being worked at elevated. temperatures to a beneficial degree. Still further reductions. on the same basis must be.v made if. more than two are present, and in alllcasesless than those maximums gives the best results. and. hardness. at. elevated. temperatures. in a molybdenum-zirconium. alloy that is well adapted to be hot-Worked, the preferred alloying transition. elementsv are titanium, vanadium, columbiumand tantalum.

Molybdenum-base alloys. characterized primarily by the. beneficial effects. of vanadium, columbium, titanium andtantalum, respectively, are. more fully disclosed; and claimed in applicants! copjending applications, filed: concurrently herewith, as follows:

Serial. No. 250,203., Molybdenum-Vanadium Alloysf; Serial No. 250,207,. Molybdenum-Columbium Alloys; Serial. No. 250,204, MolybdenumeTitanium Alloys; Serial. No. 250,205, Molybdenum-Tantalum .Alloys;

It hasbeen notedthat additions of 0.1% to .5 thorium to cast molybdenum or tungsten increase. the temperature. to which the worked metals: may, be. heated without excessive grain coarsening, and. without. becoming embrittled.

Thus; ;thoriumwithin. the range stated may be From the standpoint of high strength.

stroy the: capacity of the alloy to; be worked at;

elevated temperatures-to a beneficial degree:

Thefollowing: exemplary compositions may be successfully cast, worked at elevated. tempera tures, and have usefulpropertiesz' Erample 1.

Zirconium 1.66%.. Carbon 04%. Oxygen Less'than 005%. Molybdenum Balance;

Example 2 Zirconium 312%. Carbon .03-% (estimated). Oxygen Less than: 0.051%; Molybdenum Balance.

Erample'3' Zirconium 2%. Tungsten 5%. Carbon 025%. Oxygen Less than .0025 Molybdenum Balance.

Example 4 Zirconium 1%.. Carbon 015%.. Beryllium 02%;. Oxygen 02%.. Molybdenum Balance.

Example 5' Zirconium Aluminum 10%; Oxygen .03% Molybdenum Balance.

The alloys of this invention may. be made. by a variety of procedures, but cast alloysv contain.- ing carbon are preferably made by the process. which consists in the steps of (1) mixing molybdenum, zirconium, carbon and any other desired. elements in the form of powders, in the desired proportions; (2) pressing the mixture into successive pellets to form a continuous. rod;. (3)

sintering the rod to impart sufiicientstrengthto the same to render itself-supporting; and 4).

arc-melting the sintered rod .as a. consumableat least .01% but less than 25% is employed? The powder. chargeis fed into-an extrusion die positioned beneath the ram of a reciprocating: press wherein successive pellets ofpowd'e'r'm'aterial are pressed continuously on top of preced ing pellets to form a continuous rod of pressed metal powders. Pelletin'g pressures of approximately 10,000 p. s. i. to 20,000" ps. i. have been used, with 14,000 p. s. i. normally being adequate:

The pressing is accomplished-in avacuum-tight container.

Suflicient strength to make the pressed metal rod self-supportin is imparted by sintering the rod in vacuum at a temperature of approximately 2400" F. to 2900 F. for approximately a quarter of a minute to several minutes. Sintering may be accomplished by any well-known method of heating. Electrical resistance heating has been employed successfully.

The sintered rodis then used as a consumable electrode in a vacuum arc furnace. Melting is started by striking anarc between the rod and a starting electrode comprising a pile of chips of the same or similar alloy placed on a disc of molybdenum at the bottom of the casting mold. A water-cooled copper mold has been successfully used for receivin the molten molybdenum-zirconium alloy without contaminating the alloy with copper. Molten alloy striking the water-cooled copper mold quickly solidifies, forming a protective coating on the surface of the mold. Thereafter, the liquid alloy becomes the lower electrode and the upper, consumable electrode is mechanically fed toward the lower, liquid electrode to maintain continuous melting with proper arc spacing.

For steps 2, 3 and 4, the pressure within the container should be as low as possible and should not exceed a maximum of 500 microns and preferably should be below 100 microns. All three of these steps may be carried out in the same container.

If aluminum or beryllium or any other relatively volatile element is employed in the alloy, the above-described process cannot be practiced under the degree of vacuum set forth above and hence it is necessary to employ an inert atmosphere of, higher pressure in the melting chamber. An argon or helium atmosphere at or slightly above atmospheric pressure has been found suitable for this purpose. Except for the change from vacuum to an inert atmosphere at higher pressure, the process previously described may be used. The desired quantities of aluminum or beryllium are added to the mixture of metal powders which are sintered to produce the consumable electrode. v

Inasmuch as extremely minute quantities of oxygen impair the capacity of the alloy casting to be worked, the starting materials should be as low in oxygen as possible and it is necessary to avoid the introduction of significant amounts of oxygen as a contaminant in the inert atmosphere. The inert atmosphere may be purified by circulating it through a commercial drying tower before introduction into the casting container. The gas may be recirculated or re-used after passing over a bed of titanium metal maintained at approximately 1500 F., and a bed of magnesium metal maintained at approximately 1100 F. Because of the relatively high volatility of aluminum and beryllium at the arc temperature, the pressure of the inert atmosphere Within the casting container is preferably maintained at substantially atmospheric pressure or slightly above, for example, up to about 15.5 pounds per square inch. The casting container i first evacuated and then flushed with the inert gas; and, durin operation, the inert gas is bled into the casting container to maintain atmospheric pressure or slightly above.

If carbon is employed in addition to aluminum or beryllium, the partial pressure of carbon monoxide in the melting chamber'should be maintained below about 100 microns. In some cases, thi may require a flow of the purified gas through the chamber.

One suitable form of apparatus for use in forming, sintering and melting the powder rod is disclosed in the copending application of Edgar K. Leavenworth, Serial No. 787,797, filed November 24, 1947, now Patent No. 2,651,952.

A large number of tests with zirconium alloys having compositions within the aforementioned limits indicated that such alloys possess a tendency to retain work-hardening at elevated temperatures. The testing procedure and results are typified by the following data. The alloy of Example 2 in the form of a bar 1 /2 inches in diameter and 2 inches long after heating in the range of 2500 F. to 2600 F. was extruded in a die having a diameter of .78 inch. After extrusion, the hardness Was 320 V. P. N. at room temperature. After the gas-extruded bar was annealed at 2200 F. for 1 hour, the hardness was 295 V. P. N. After 1 hour at 24=00 F., the hardness was 247 V. P. N. This alloy, in an annealed casting, had a hardness of 24.0 V. P. N. The beneficial tendency of zirconium to retain work-hardening at elevated temperatures is apparent when it is noted that, in the absence of zirconium, molybdenum similarly treated and having the same carbon content lost all of its work-hardness in 1 hour at 2000 F.

Microscopic examination of a large number of molybdenum-zirconium alloys showed that the addition of zirconium redistributes the carbide phase, leaving little carbide segregation at the grain boundaries. The redistribution of the carbide phase improves the ability of the alloy bon, oxygen 'less than .02%, and the balance consisting essentially of molybdenum.

2. A cast alloy consisting of at least molybdenum and characterized by its capacity to be Worked at elevated temperatures and its capacity to retain a significant amount of work hardness after one hour at 2200 F., said alloy containing from .1% to 2.0% zirconium; at least one element from the group consisting of carbon from 01% to 25% aluminum from 003% to 2.5% and beryllium from .001% to 25%; metal from the group consisting of the transition elements titanium from zero to 14%, vanadium from zero to 7%, chromium from zero to 2%, iron from zero to 1.3%, cobalt from zero to .9%, nickel from zero to 4%, columbium from zero to 10%, tantalum from zero to 9%, and tungsten from zero to 10%, the total amount of metal from said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600 F.; and the balance consisting essentially of molybdenum.

3. A cast alloy consisting of at least 85% molybdenum and characterized by its capacity to be worked at elevated temperatures and its capacity to retain a significant amount of work hardness after one hour at 2200 F., said alloy containing from .1% to 2.0% zirconium; from bon; oxygen less than :02-%; metal from the group consisting of the" transition elements titanium from :zero to 14%,vanadium from zero to. 7%, chromium from zero to 2%, iron from zero to 10%., the total amount of metal from To said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600 F.; and the balance consisting essentially of molybdenum.

4. A cast alloy consisting of at least 85% molybdenum and characterized by its capacity to be worked at elevated temperatures, said alloy containing from .1% to 2.0% zirconium; at least one element from the group consisting of carbon from .01% to 25%, aluminum from 003% to 2.5% and beryllium from .001% to .25%; metal from the group consisting of the transition elements titanium from zero to 14%, vanadium from zero to 7%, chromium from zero to 2%, iron from zero to 1.3%, cobalt from zero to 9%, nickel from zero to .4%, columbium from zero to 10%, tantalum from zero to 9%, and tungsten from zero to 10%, the total amount of metal from said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600 F.; and the balance consisting of molybdenum.

5. A cast alloy characterized by its capacity to be worked at elevated temperatures and its capacity to retain a significant amount of work hardness after one hour at 2200" F., said alloy casting comprising from .1% to 2.0% zirconium; carbon from .01% to 25%, oxygen not more than 005%, and the balance consisting essentially of molybdenum.

6. A cast alloy characterized by its capacity to be worked at elevated temperatures and its capacity to retain a significant amount of work hardness after one hour at 2200 F., said alloy casting comprising from .01% to 2.0% zirconium, oxygen not more than 05%, aluminum in an amount at least sufiicient to react with all of the oxygen present and not more than 2.5%, the maximum amount of aluminum within the range stated beingreduced toward 4% as the amount of zirconium approaches its upper limit, and the balance consisting essentially of molybdenum.

7. A cast alloy characterized by its capacity to be worked at elevated temperatures and its capacity to retain a significant amount of work hardness after one hour at 2200 F., said alloy casting comprising from .1% to 2.0% zirconium; oxygen not more than 05%, beryllium in an amount at least sufficient to react with all of the oxygen present and not more than 25%, the maximum amount of beryllium within the range stated being reduced toward 03% as the amount of zirconium approaches its upper limit, and the balance consisting essentially of molybdenum.

8. A cast alloy consisting of at least 85% molybdenum and characterized by its capacity to be worked at elevated temperatures, said alloy casting containing from .l% to 2.0% zirconium; carbon from .01% to 25%, oxygen not more than 005%, metal from the group consisting of the transition elements titanium from zero to 14%, vanadium from zero to 7%, chromium from zero to 2%, iron from zero to 1.3%, cobalt from zero to .-9%,' nickel from zero to .-4%, columbium from zero to 10%, tantalum from zero to 9%, and tungsten from zero to 10%, the total amount of metal from said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600 F., and the balance consisting of molybdenum.

9. A cast alloy consisting of at least 85% molybdenum and characterized by its capacity to be worked at elevated temperatures, said alloy casting comprising from .1% to 2.0% zirconium; oxygen not more than .05%, aluminum in an amount at least sufficient to react with all of the oxygen present and not more than 2.5%, the maximum amount of aluminum within the range stated being reduced toward 04% as the amount of zirconium approaches its upper limit, metal from the group consistin of the transition elements titanium from zero to 14%, vanadium from zero to 7%, chromium from zero to 2%, iron from zero to 1.3%, cobalt from zero to .9%, nickel from zero to .4%, columbium from zero to 10%, tantalum from zero to 9%, and tungsten from zero to 10%, the total amount of metal from said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600 F., and the balance consisting of molybdenum. v

10. A cast alloy consisting of at least 85% molybdenum and characterized by its capacity to be worked at elevated temperatures, said alloy casting comprising from .1% to 2.0% zirconium; oxygen not more than .05%, beryllium in an amount at least sufficient to react with all of the oxygen present and not more than .25%, the maximum amount of beryllium within the range stated being reduced toward .03% as the amount of zirconium approaches its upper limit, metal from the group consisting of the transition elements titanium from zero to 14%, vanadium from zero to 7 chromium from zero to 2%, iron from zero to 1.3%, cobalt from zero to 9%, nickel from zero to .4%, columbium from zero to 10%, tantalum from zero to 9%, and tungsten from zero to 10%, the total amount of metal from said group of transition elements being further limited to an amount within the range from none to the amount which will increase the hardness of the annealed casting to a value not exceeding 200 V. P. N. at 1600" F., and the balance consisting of molybdenum.

11. A cast, molybdenum-base alloy characterized by its capacity to be worked at elevated temperatures, said alloy casting comprising from .1% to 2.0% zirconium, carbon from .01% to 07%, oxygen not more than 003%, and the balance consisting of molybdenum.

12. A cast, molybdenum-base alloy characterized by its capacity to be worked at elevated temperatures, said alloy casting comprising from .1% to 2.0% zirconium, aluminum from 003% to .4%, carbon not more than 06%, oxygen not more than 05%, the minimum amount of aluminum within the range stated being that required to combine with all of the oxygen in the alloy to form aluminum oxide, and the balance consisting of molybdenum.

13. A cast, molybdenum-base alloy characterized by its capacity to be worked at elevated temperatures, said alloy casting comprising from .1% to 2.0% zirconium, beryllium from .001% to .03%, carbon not more than .06%, oxygen not more than .05%, the minimum amount'of beryllium within the range stated being that required to combine with all of the oxygen in the alloy to form beryllium oxide, and the balance consisting of molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS Number OTHER REFERENCES Parke et al.: Treatise in Transactions of Mining and Metallurgical Engineers, vol. 171, 1947, pages 416-430.

Kessler et al., 1949. Preprint No. 33 of paper 10 presented at the American Society for Metals Convention, Cleveland, Ohio, October 1'7-21, 1949. 

1. A CAST ALLOY CONSISTING OF AT LEAST 85% MOLYBDENUM AND CHARACTERIZED BY ITS CAPACITY TO BE WORKED AT ELEVATED TEMPERATURES AND ITS CAPACITY TO RETAIN A SIGNIFICANT AMOUNT OF WORK HARDNESS AFTER ONE HOUR AT 2200* F., SAID ALLOY CONTAINING FROM .1% TO 2% ZIRCONIUM, FROM .003% TO .4% ALUMINUM, FROM ZERO TO .02% CARBON, OXYGEN LESS THAN .02%, AND THE BALANCE CONSISTING ESSENTIALLY OF MOLYBDENUM. 